Recently, I received a copy of Neil Shubin’s Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. Shubin, you may recall, is one of the co-discoverers of Tiktaalik roseae.
In Your Inner Fish Shubin sets out to show how many features of the human body can be traced back to fish, worms, and flies, among other organisms. The book is targeted for a popular audience and as such is written in a highly accessible style. The book is short, coming in at 201 pages (not including references, acknowledgments, and the index), and is divided into ten chapters and an epilogue. Each chapter walks you through how some feature of human anatomy can be traced back to fish or worms or what have you. One example will have to suffice.
Chapter five, appropriately called “Getting Ahead”, begins with Shubin studying cranial nerves several days before an anatomy test. Most cranial are easy because they have only one function and attach to one muscle or organ. Four, however, are a bit more difficult to trace. Shubin focuses on two of the four – the trigeminal and the facial. Each breaks up into a number of smaller branches that take a complex path through the head. The trigeminal nerves controls some of the muscles we use for chewing, innervate teeth, control some muscles in the inner ear, and is responsible for facial sensation. The facial nerve controls the muscles used in making facial expressions and like the trigeminal, it also controls some muscles in the inner ear. The question is why? As Shubin puts it:
Nothing about them seems to make any sense. For example, both the trigeminal and the facial nerves send tiny branches to muscles inside our ears. Why do two different nerves, which innervate entirely different parts of the face and jaw, send branches to ear muscles that lie adjacent to one another? Even more confusing, the trigeminal and facial almost crisscross as they send branches to our face and jaw. Why?
To answer that we need to look at the developing embryo. Part of the answer lies in the four arches that for roughly 3-4 weeks after conception. The first arch tissues, ultimately, form the upper and lower jaws, the malleus and incus, and all the muscles that supply them. The second arch forms the stapes, the hyoid and the muscles that control facial expression. The trigeminal comes from the first arch, while the facial nerve comes from the second. But there is more. In sharks, the first arch forms the jaws, which are enervated by the trigeminal. The second arch, in sharks, forms a cartilage rod that eventually breaks up to form to bones that support the jaws. The first bone is the equivalent to the hyoid in humans and supports the lower jaw. The second bone, which supports the upper jaw is equivalent to the stapes in humans. oing a step further, if we look at Amphioxus we can see a notochord. Humans have one too, but ours breaks up early in embryology and, according to Shubin, becime part of the intervertebral disks. Like humans, Amphioxus also has arches. In this case, the arches form cartilage that support the gill slits. Going back in time, say 500 million years, we can see the notochord preserved in some of the earliest worm fossils.
This is brief overview of one chapter demonstrates one of the strengths of Shubin’s book. Shubin starts with an anatomical puzzle, compares the anatomy to that seen in other organisms, discusses the developmental aspects, and then brings in the paleontological record to support his points. This is a powerful approach to doing science, and a powerful approach to explaining science. Overall, the book is an enjoyable read and I would recommend it to anyone interested in science.
Finally honesty* compels me to issue a disclaimer. In the online resources section of the Shubin does recommend my blog, along with several others, as being one worth following for information and commentary on recent discoveries. This mention, while gratifying, does not impact my opinion of the book. Finally, here is an audio of an interview with Shubin in conjunction with the release of the book
* Okay, I’m bragging a little because that is the first time I have seen my blog mentioned in a book.